1. Field of the Invention
The present invention relates to a pinless pressure foot for a machine tool, such as a router, and, more particularly, to a simpler, less costly, easier to maintain, and otherwise more efficient pinless pressure foot for a machine tool.
2. Description of the Prior Art
In the operation of automatic routing machines, it is conventional to position a stack of workpieces, such as those used in producing printed circuit boards, on a worktable and to hold them in place by tooling pins which extend through openings in the workpieces. For accuracy, there is a close fit between the tooling pins and the workpieces at the openings so that there is no clearance around the pins to permit relative lateral movement. Because of this relatively tight fit, the process of inserting the tooling pins into the stacked workpieces is laborious and time consuming.
It has also been necessary to provide these tooling pins in the portions of the workpieces which are cut out, as well as the portions outside of the cut line, because otherwise, the cutout portions will be moved laterally during the cutting operation and an accurate cut will not be produced. This lateral movement of the cutout portion will occur during the last increment of the cut, arising from the lateral force produced by the router bit as the worktable is moved relative to the spindle. Prior to that time, the portion of the workpiece to be cutout will be joined to the portion outside of the cut line by sufficient material to prevent relative movement. When the cut is nearly complete, however, the connecting portion becomes very small and lacks the strength to withstand the lateral force of the router. Thus, unless it has been held by tooling pins or other auxiliary clamping members, the cutout portion will break off or move as the connecting portion is cut through and its edge will not be accurately formed.
The necessity for using tooling pins in the cutout portions has substantially increased the production time for the machine. The drilling of the various holes for the tooling pins and the close tolerances required take a substantial amount of the operator's time in preparing the workpieces for routing and installing them on the worktable. After the routing operation is complete, additional time is expended in prying the cutout parts off of the tooling pins that hold them. Consequently, the production rate of a routing machine has been slowed and expenses have been increased because of the need for pinning the portions of the workpieces to be cut out.
In order to eliminate the necessity for tooling pins in the cutout portions, it has been proposed to provide means which will hold the cutout portions of the workpieces without the use of such tooling pins or other auxiliary clamps. This materially improves the efficiency of the routing machine and production rates are increased. The means for holding the cutout portions, referred to as a pinless pressure foot, consists of a workpiece engagable means circumscribing the cutting tool of the routing machine and being movable axially relative to the spindle for selectively engaging the surface of the portion of the workpiece, forcing the portion of the workpiece against the worktable for preventing movement thereof relative to the worktable during completion of the cut. Prior to completion of the cut, the workpiece engagable means moves with the spindle, above the workpiece. When the cutting tool has reached a predetermined point prior to completion of the cut, a drive mechanism moves the workpiece engagable means into contact with the workpiece and the workpiece engagable means remains stationary as the cutting tool moves laterally to complete the cut. When the cut has been completed, the workpiece engagable means is released and returned to its centered position relative to the spindle.
In prior art pinless pressure feet, there has been provided a housing around the spindle and the router bit, at the lower portion of which is a slide member which is movable laterally relative to the housing. This movement has ordinarily been prevented by a locking pin arrangement, including multiple locking pins which may be selectively released to release the slide member. Carried by the slide member is a sleeve that circumscribes the router bit and includes an annular member that is held against the surface of the workpiece during routing. This is accomplished by a downward force on the housing. The annular member may be a brush arrangement which produces a relatively low friction drag. Its downward force on the workpiece holds the workpiece flat against the worktable as the worktable moves relative to the spindle in producing the cut in the workpiece, compensating for warpage of the workpiece. This annular member also forms an enclosed chamber to which a vacuum line may be connected to remove the debris generated during the routing operation.
Also included on the slide member is an element having a relatively high coefficient of friction, such as an O-ring, which is normally spaced above the surface of the workpiece. However, when the cut is nearly completed, an increased force is applied to the housing, urging it and the slide member downwardly relative to the spindle, causing the O-ring to be brought into engagement with the surface of the workpiece. The engagement of the O-ring with the workpiece is around the router bit so that the O-ring engages the portion of the workpiece within the cut line as well as outside of the cut. Simultaneously with the forcing of the housing downwardly, the locking pin arrangement is withdrawn so that the slide member can move laterally relative to the housing. Lateral movement of the slide member relative to the housing is permitted either by a ball bearing arrangement, an air bearing or another low friction arrangement between the slide member and the housing. Accordingly, as the worktable proceeds to move laterally relative to the router for completing the cut, the slide member remains stationary relative to the worktable and, through the O-ring, holds the cutout portion of the workpiece motionless relative to the worktable. As this takes place, the slide member moves laterally relative to the spindle.
Upon completion of the routing operation, the head of the machine is raised, lifting the housing and associated elements away from the workpiece, with the cutout portion then being free for removal from the machine. Typically, centering springs return the slide member to its original position and the locking pin arrangement is activated preparatory to the next cutting operation.
While such a pinless pressure foot materially improves the efficiency of routing machines, several problems remain. The housing and the slide member and all of the mechanism for activating same are all typically located in the crowded area surrounding the cutting tool which creates a number of problems. Because this is the area where the tool and the work is, the mechanism interferes with the work area of the operator and is subject to contamination from the routing operation. By being located in the work area, the pinless pressure foot is subject to damage, increasing maintenance costs. The mechanism is also less accessible for repair.
The force for moving the slide member into contact with the workpiece is typically generated by multiple cylinder/piston arrangements surrounding the spindle. If these cylinders/pistons are not exactly balanced, binding of the movable housing may occur. Similar problems occur with regard to the locking pin arrangement which typically includes multiple cylinders/pistons. Because of the multiple drive means for the locking pin arrangement and the drive means, the mechanism is complex, expensive, and time consuming to maintain.
The method for permitting lateral movement of the slide member relative to the housing has always included a bearing located within the vicinity of the workpiece, where it is subject to contamination and damage. Separate means must be provided for centering of the slide member relative to the housing, adding to the complexity and cost of the system.